For example, methods of manufacturing a hollow pipe-body from a metal plate are disclosed in the specifications of Japanese Patent No. 3114918 (Patent Document 1), Japanese Patent No. 5886325 (Patent Document 2), Japanese Patent No. 3974324 (Patent Document 3), and Japanese Patent No. 4040840 (Patent Document 4).
In the manufacturing method disclosed in Patent Document 1, a shape which is curved in a longitudinal direction is formed in a first forming process, and a pipe-shaped cross section is formed in a subsequent forming process.
In the manufacturing method disclosed in Patent Document 2, similarly to in Patent Document 1, a shape which is curved in a longitudinal direction is formed in a first forming process, and a pipe-shaped cross section is formed in a subsequent forming process. A core is used as a tool to process one longitudinal direction portion into a rectangular cross-section.
In the manufacturing methods disclosed in Patent Document 3 and Patent Document 4, in a first processing step, a wall portion with a seam is formed in a finished pipe-body. In a second processing step, an angle between one wall portion, which faces the seam, and another wall portion, which is adjacent to the one wall portion, is configured to be larger than an angle of the finished pipe-body.
In a third processing step, a protruding curved surface, which protrudes toward the outside, is formed at the one wall portion by an external force applied to the other wall portion. Then, in a fourth processing step, the protruding curved surface is deformed and flattened by an external force applied to the one wall portion, and the pipe-body in which a pair of edges contact each other closely is formed by spring-back force.